1. Field of the Invention
This invention relates to an invertor-controlled power unit incorporating an invertor device, which is driven by an engine to supply AC power.
2. Prior Art
In recent years, portable AC power units have been increasing in number, which are equipped with invertor devices for stabilizing the output frequency. For example, a portable power unit which generates AC power having a commercial frequency by means of an AC generator driven by an engine is known, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 59-132398, in which the engine is operated within a high speed range to cause the AC generator to generate high AC output, the output is converted into direct current, then the direct current is converted into alternating current by an invertor device, and the alternating current is outputted.
It has been also proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-114527, to additionally provide such a portable invertor-controlled AC power unit with an excessive current protection device which is adapted for use in a portable power unit having a relatively small capacity.
However, if it is required to provide an AC power unit of this type with means for cutting off a leak, means for detecting a leak has to be provided in addition to the excessive current protection device, since the amount of leak current to be detected is often relatively small in such a small capacity power unit. Therefore, in most cases, an external earth leakage breaker is used to cope with a leak in the portable AC power unit of this type.
However, a portable AC power unit having an external earth leakage breaker suffers from problems of an increased size, overlapping functions, and a high cost.
To solve these problems, the present assignee has proposed an invertor-controlled power unit in Japanese Patent Application No. 2-333714, which employs a current transformer for leak detection. The proposed power unit is very excellent in that it has a leak detecting construction which is small in size, light in weight and low in manufacturing cost, and it is able to cut off a leak in quick response to the leak. However, since the power unit uses the current transformer, it is difficult to incorporate leak detection/cutting-off means onto a circuit board, as well as further decrease the size and cost.